Two armies are preparing to attack a city from opposite sides. The General of army A is orchestrating the attack, and has decided that they must attack simultaneously at noon on March 3rd to succeed. He sends a sealed, encrypted letter by messenger across the valley to the General of army B, informing him of the plan, but worries that the messenger might be intercepted, so General B may not be informed. (If the attack is not simultaneous, both attacking armies will be destroyed by the defenders.)

As it so happens, General B does receive the message, but knows that General A cannot be sure of this. He sends back a receipt of the attack plan. Then he wonders... will it go through? What if General A does not receive it and decides not to attack, being unsure of B's knowledge? And even if General A sends over an acknowledgment of the plan's receipt, there is no guarantee it will arrive.

Given this faulty communication channel, how will the generals coordinate their plan?